The absolute flux calibration of the James Webb Space Telescope will be based on a set of stars observed by the Hubble and Spitzer Space Telescopes. In order to cross-calibrate the two facilities, several A, G, and white dwarf (WD) stars are observed
with both Spitzer and Hubble and are the prototypes for a set of JWST calibration standards. The flux calibration constants for the four Spitzer IRAC bands 1-4 are derived from these stars and are 2.3, 1.9, 2.0, and 0.5% lower than the official cold-mission IRAC calibration of Reach et al. (2005), i.e. in agreement within their estimated errors of ~2%. The causes of these differences lie primarily in the IRAC data reduction and secondarily in the SEDs of our standard stars. The independent IRAC 8 micron band-4 fluxes of Rieke et al. (2008) are about 1.5 +/- 2% higher than those of Reach et al. and are also in agreement with our 8 micron result.
Absolute flux distributions for seven solar analog stars are measured from 0.3 to 2.5 mu m by HST spectrophotometry.In order to predict the longer wavelength mid-IR fluxes that are required for JWST calibration, the HST SEDs are fit with Castelli & K
urucz model atmospheres; and the results are compared with fits from the MARCS model grid. The rms residuals in 10 broad band bins are all <0.5% for the best fits from both model grids. However, the fits differ systematically: The MARCS fits are 40-100 K hotter in T_{eff}, 0.25-0.80 higher in log g, 0.01-0.10 higher in log z, and 0.008-0.021 higher in the reddening E(B-V), probably because their specifications include different metal abundances. Despite these differences in the parameters of the fits, the predicted mid-IR fluxes differ by only ~1%; and the modeled flux distributions of these G stars have an estimated ensemble accuracy of 2% out to 30 mu m.
Absolute flux distributions for eight stars are well measured from 0.8-2.5mu m with NICMOS grism spectrophotometry at a resolution of R~100 and an accuracy of 1-2%. These SEDs are fit with Castelli & Kurucz model atmospheres; and the results are comp
ared with the Cohen-Walker-Witteborn (CWW) template models for the same stars. In some cases, the T_{eff}, log g, and log z parameters of the best fitting model differ by up to 1000 K from the earlier CWW model. However, differences in the continua of the modeled IR flux distributions from 0.4-40mu m are always less than the quoted CWW uncertainty of 5% because of compensating changes in the measured extinction. At wavelengths longward of the 2.5mu m NICMOS limit, uncertainties still approach 5%, because A-star models are not yet perfect. All of these A stars lie in the JWST continuous viewing zone and will be important absolute flux standards for the 0.8-30mu m JWST wavelength range.
